The pursuit of peace has remained a central aspiration throughout human history, reflected across philosophical, spiritual, and socio-political traditions worldwide. The Upanishads, regarded as the philosophical culmination of the Vedic corpus, present a unique and holistic conception of peace—not merely as the absence of conflict, but as an inner state of harmony that radiates outward to encompass society and the cosmos. This paper offers a critical philosophical inquiry into the significance of Shanti Mantras (peace invocations) within key Upanishadic texts, including the Isha, Taittiriya, Mandukya, Kena, and Mundaka Upanishads. Through a close textual and thematic analysis of mantras such as “ॐ शान्तिः शान्तिः शान्तिः,” “ॐ सह नाववतु...,” and “ॐ द्यौः शान्तिः...”, the study elucidates a threefold vision of peace—adhyatmika (personal), adhibhautika (environmental), and adhidaivika (cosmic)—emphasizing that sustainable peace begins with inner transformation. Further, this paper explores the philosophical foundations of these mantras through the lens of key Indian concepts such as Ahimsa (non-violence), Samatva (equanimity), and Atma-Brahma unity (the oneness of self and ultimate reality). Drawing upon classical commentaries and contemporary applications, the research highlights how these ancient teachings continue to inform modern practices in meditation, therapy, wellness, and interfaith peacebuilding. In light of escalating global anxiety, ecological crises, and socio-political unrest, this study argues that the Upanishadic worldview offers a valuable alternative framework for cultivating holistic peace, grounded in spiritual awareness, ethical conduct, and universal harmony. Ultimately, the study bridges ancient Indian wisdom with contemporary peace studies, demonstrating the enduring practical relevance of Shanti Mantras for fostering well-being within individuals, societies, and the global community.